Steel heating boilers of the above-mentioned type are generally known. To enable heating boilers to be operated with smoothly variable temperatures, it has also become known to provide the installation unit on the burner and fuel-gas sides with corrosion-resistant linings consisting, for example, of enamel suitable for this purpose.
Apart from the fact that heating boilers, where the spaces carrying the burner flame and fuel gases form an installation unit, can be manufactured very economically under certain preconditions, such installation units designed very simply in terms of spatial relationships are, of course, also very advantageous from the point of view of the application or coating of a corrosion-preventing or enamel lining, as defined in Volume 4, Sanitary and Heating Engineering, 1965, page 316, as corrosion protection in boiler construction.
However, in such constructions, the closures at the front and/or rear at the entrances to the tubular installation unit are of critical importance, since the condensate occurring unavoidably can, on the one hand, attack the closure seals and/or leak out and then damage the outer faces of the boiler. On the other hand, there is the difficulty of welding into the housing the unit which must receive its corrosion-resistant lining before installation in the water-bearing housing, without damaging the lining during welding.